342 
HARRY R. ROSEN 
of the upper and lower epidermis of the leaf adjoining the midrib are 
the results of the first twenty-four hours of insect attack. 
Concerning the histological structure and development of the gall, 
brief descriptions are given by Cook (4), Pantanelli (22), and Cornu 
(8 and 9). As already pointed out, gall formation starts on the young 
bud leaf, where the insect usually places herself along a primary vein, 
either directly on top of the vein or directly alongside of it, and an 
abnormally large number of unicellular and multicellular hairs, grow 
up around her from the upper epidermis. The lower epidermis like- 
wise feels the stimulus and the abnormal production of similar hairs is 
started. The tissue beneath the insect appears sunken so that a 
basin-like cavity is produced on the upper surface of the leaf, in which 
the insect lies. Figure 2 shows the insect drawn away from the hollow 
in which it was resting, but a portion of its proboscis is still seen 
piercing the tissue of the vein. ' 
The apparent sinking in of the upper tissues of the leaf is due to a 
partial collapse of the young mesophyll tissue. Figure 9 shows a 
cross section of a young bud leaf. The proboscis of the insect is seen 
piercing through the epidermal cells, and appears broken off from the 
rest of the insect, which is drawn away from the leaf. The proboscis, 
curved around and reaching to the phloem of the vein, was followed in 
three successive sections, only one of which is shown. The narrowness 
of the leaf where the setae of the insect are projecting, and the papil- 
late projections bordering the hollow are worthy of note. The con- 
stricted portion measures 72^1 as compared to the normal unconstricted 
portion, which measures 96 fi. Comparison of the cells in the two 
regions shows that this difference in size between the constricted 
portion and the unconstricted portion is due to the difference in size of 
the mesophyll which measures 40 /jl at the constricted part, where the 
proboscis protrudes, as compared with the normal unconstricted 
mesophyll which measures 68 microns. Thus we see that the first 
twenty-four hours of insect attack brings forth a decrease in size of the 
portion attacked, accompanied by the production of hairs around the 
constricted portion, which is a hypertrophic effect. Reasons for these 
developments will be discussed later. 
Following the histological development of the gall, we find after 
three to four days of ^insect attack, that rapid proliferation of the 
underside of the leaf begins to take place, while the tissue of the upper 
side shows a slight enlargement of the palisade cells and of the upper 
